Rustless and refractory bimetallic strip for high temperatures



y 1962 A. G. CHERREAU ETAL 3,045,335 RUSTLESS AND REFRACTORY BIMETALLICSTRIP FOR HIGH TEMPERATURES Filed Nov. so, 1959 20 Fig. 1

i F' g 2 20 i Fig. 3

United States Patent 3,045,335 RUSTLESSS AND REFRACTORY BIMETALLEC STlFGR HllGll-ll TEMPERATURES Andre Gaston Cherreau, Employ, Andre ReneMichel Girard, Nevers, and Guy Lucien Robert Maingault, Imphy, France,assignors to Societe Metallnrgique dlmphy, Paris, France, a company ofFrance Filed Nov. 30, 1959, Ser. No. 856,111 Claims priority,application France Dec. 4, 1958 6 Claims. (Cl. 29-1955) NumerousVarieties of bimetallic strips are known. These strips are formed by theassociation of two strips closely joined to each other, whosedilatabilities are different enough to allow the strip to incurve underthe effect of temperature changes.

The extent of the distortion of the bimetallic strip depends on itssensitiveness, which is defined by the relation:

V is the Villarceaus coefficient.

a and a the dilatation coefficients of the most dilatable and the leastdilatable constituents.

The bimetallic strip effect enables a displacement to be obtained and astress to be developed when there are temperature variations, it isfrequently utilized in recording, regulating or safety devices.

The evolution of techniques entails more and more severe requirements,and sets new problems. It is necessary that bimetallic strips:

Can be utilized at relatively high temperatures,

Have a deflection proportional to the differences in temperaturethroughout the entire field of temperatures used. In case, thedeflection-temperature curve is a straight line,

Should resist somewhat injurious atmospheres,

Should allow the most diverse parts to be made, thanks to a satisfactorymalleahility and especially an excellent aptitude for bending, Whileretaining sufficient mechanical characteristics,

Should have a perfect physico-chemical stability.

The field in which known bimetallic strips can be employed is generallylimited to temperatures in the region of 400 to 500 centigrade, and notone of them complies with the above-mentioned requirements.

The present invention completes the range of existing bimetallic strips.

It must be borne in mind that the mere knowledge of each of the twoalloys forming a bimetallic strip does not enable the properties of thebimetallic strip to be prejudged. Actually, it is indispensable, toobtain perfect functioning, that the transition zone between the twoconstituents assembled hota zone whose chemical composition ismixedshould possess mechanical and dilatometiical properties which donot run counter to forecasts. For example, it would seem at first sightthat by selecting iron or mild steel as the slightly dilatableconstituent, and as the most dilatable constituent the well-knownferrous alloy of 18% Cr and 8% Ni, we can have a normally utilizablebimetallic strip. Now, this is not so, because the welding not givesrise, between the ferri-tic and austenitic constituents, to anintermediate martensitic zone which is unstable and brittle. Researchwork and experiments have alone been able to show that this disadvantagedoes not exist with the bimetallic strip forming the subject of theinvention. This bimetallic strip is rustless, useful at hightemperatures and has a proportional deflection. It can be used up to 650centigrade and formed by a slightly dilatable element of a ferrous alloyessentially of copper-chrome, containing 18 to 25% 3,045,335 PatentedJuly 24, 1962 CC V Cr and, 0.3 to 2.5% Cu, associated wtih a highlydilatable alloy element formed essentially of iron, nickel and chromecontaining 40 to 50% Ni and 17 to 28% Cr. Moreover, the slightlydilatable alloy may contain a maximum of 0.8% C, 1% Si, 1% Ni, 1.5% Mnand the highly dilatable alloy, a maximum of 0.5% C and 1.5% Mn with 0.5to 3% Si.

Furthermore, these two alloys have the usual contents of impurities.

FIGURE 1 attached gives the dilatation curves of the two alloysmentioned above.

FIGURE 2 reproduces in function of the temperature shown in abscissae,the deflection curve of the bimetallic strip obtained by the associationof the above-mentioned alloys, this deflection being the displacementmeasured in millimetres of the free end of a bimetallic strip 100 mm.long and 1 mm. thick embedded in the other end.

The bimetallic strip according to the invention complies with therequirements demanded.

Its deflections are proportional to temperatures up to 650 centrigrade.

It possesses the following physical and mechanical properties:

Sensitiveness Villarceaus coefiicient: V=7 10- (which corresponds to aspecific deflection of 0.035)

Resistivity: By way of information, microhms-cm.

Elasticity moduli centigrade):

At 20 kg./mrn. 19,500 At "kg/mm? 19,000 At 200 "kg/mm?" 18,400 At 300kg./mm. 17,800 At 400 kg/mm?" 17,100 At 500 kg./mm. 16,500 At 600"kg/mm? 12,100

Mechanical characteristics:

After annealing for half an hour at 650 C. and air c0oling- Elasticlimit kg/mm?" 73 Breaking load kg./mm. 85 Elongation "percent" 9 Afterannealing for half an hour at 700 C. and air cooling- Elastic limitkg./mm. 60 Breaking load kg/mm?" 75 Elongation percent 14 Permissiblefatigue rate (bimetallic strip annealed for /2 hr. at 650):

At the ambient temperature, S:

about kg./mm. 25 At 300, S=about "kg/mm? 10 At 500, S=about kg/mm? 1.5At 600, S= about kg./mm. 0.7

Thus, the scope of the temperature employed may be extended up to 650centigrade when no stress is required, and preferably up to 600centigrade when the bimetallic element must exert a mechanical stress ofappreciable value.

In actual practice, certain kinds of bimetallic strips for which themaximum working temperature is 500 centitures, because its sensitivenessremains constant up to its maximum working temperature.

The constituents are high temperature alloys resisting oxidation andsulphurous atmospheres.

After annealing at 650 to 700 C. for at least half an hour, thephysico-chemical stabilization of the constituents is perfect and itsmechanical characteristics are equal to or exceeding that of bimetallicstrips that must be used after a low temperature stoving. In particular,the elongation is suflicient to allow of complicated shapes to beobtained, without the risk of surface cracks forming.

The properties stated above for the bimetallic strip according to theinvention are most invaluable in certain cases:

As a first example, we would mention the making of discs stamped in theshape of a spherical cap which must be turned over when the temperaturereaches a relatively high given value. It is known that the displacementof the summit of the spherical cap in relation to its initial positionoccurs by an affected hysteresis cycle, as shown in the example given inFIGURE 3, which gives, in function of the temperature, the displacementin millimetres of the summit of a spherical cap. If the kind ofbimetallic strip, the thickness and diameter of the disc, are carefullychosen, it is only necessary to adjust the stamping depth so that theequilibrium of the tensions causes the abrupt turning over of thespherical cap for heating or cooling at the required temperature.

With known bimetallic strips, it is not possible to produce parts thatturn over at a relatively high temperature and possessing a satisfactorydifference between the turning over temperatures at heating or cooling,because:

Their field of employment is limited to temperatures in the region of500 C.,

Their parasite tension cannot be eliminated by mere stoving,

Their high sensitiveness leads to too great a stamping depth.

The new bimetallic strip, which can be subjected to real annealing,enables the problem to be easily solved. The weakness of its Villarceauscoefiicient, compared with that of other metallic strips, becomes aninteresting characteristic.

Another example in which the bimetallic strip according to the inventionenables a distinct improvement to be obtained is that of regulating gasappliances of all kinds, in which the bimetallic strips are liable to bespeedily deteriorated by flames.

Although the whole of the characteristics stated can be obtained byutilizing the entire field of the compositions that have been mentioned,it is generally preferable to employ the following compositions:

For the slightly dilatable alloy- Cr, 20 to 25% Cu, 0.3 to 2% For thehighly dilatable alloy Ni, 43 to- 48% Cr, 20 to 26% Within these limits,the following examples are especially interesting:

For each of the above-mentioned examples, the bimetallic strip wasannealed at a temperature of 650 to 700 for at least half an hour.

We claim:

1. Rustless, high temperature bimetallic strip with proportionaldeflection utilizable up to 650 C., formed by a slightly dilatableelement combined with a highly dilatable element, both of a ferrousalloy, characterised by the fact that the slightly dilatable elementconsists essentially of 18 to 25% of chromium and 0.3 to 2.5% of copperand balance Fe, and that the highly dilatable element consistsessentially of 40 to 50% of nickel and 17 to 28% of chromium and balanceFe.

2. Bimetallic strip according to claim 1, characterised by the fact thatthe slightly dilatable element includes a maximum of 0.8% of carbon, 1%of silicon 1% of nickel and 1.5% of manganese.

3. Bimetallic strip according to claim 1, characterised by the fact thatthe highly dilatable element includes a maximum of 0.5% of carbon, 1.5%of manganese and from 0.5% to 3% of silicon.

4. Bimetallic strip according to claim 1, characterised by the fact thatthe slightly dilatable element consists essentially of from 20 to 25% ofchromium and from 0.3 to 2% of copper and balance Fe, and that thehighly dilatable element consists essentially of from 43 to 48% ofnickel and from 20 to 26% of chromium and balance Fe.

5. Rustless, high temperature bimetallic strip with a proportionaldeflection utilisable upto 650 C., formed of a slightly dilatableelement combined with a highly dilatable element, both of a ferrousalloy, characterised by the fact that the slightly dilatable elementconsists essentially of 0.24% of carbon, 0.40% of silicon, 0.40% ofmanganese, a maximum of 0.40% of nickel, 21.5% of chromium and 1.2% ofcopper and balance Fe, and that the highly dilatable element consistsessentially of a maximum of 0.10% of carbon, 2% of silicon, 1.2% ofmanganese, 46.5% of nickel and 23% of chromium and balance Fe.

6. Rustless, high temperature bimetallic strip with a proportionaldeflection utilizable up to 650 C., formed of a slightly dilatableelement combined with a highly dilatable element, both of a ferrousalloy, characterised by the fact that the slightly dilatable elementconsists essentially of 0.10% of carbon, 0.20% of silicon, 0.30% ofmanganese, a maximum of 0.40% of nickel, 25 of chromium, and 0.4% ofcopper and balance Fe, and that the highly dilatable element consistsessentially of a maximum of 0.10% of carbon, 1.7% of silicon, 0.8% ofmanganese, 46% of nickel and 21% of chromium and balance Fe.

References Cited in the file of this patent UNITED STATES PATENTS1,991,438 Wohrman Feb. 19, 1935 2,461,518 Chace Feb. 15, 1949 2,770,870Mooradian Nov. 20, 1956

1. RUSTLESS, HIGH TEMPERATURE BIMETALLIC STRIP WITH PROPORTIONALDEFLECTION UTILIZABLE UP TO 650*C., FORMED BY A SLIGHTLY DILATABLEELEMENT COMBINED WITH A HIGHLY DILATABLE ELEMENT, BOTH OF A FERROUSALLOY, CHARACTERISED BY